Self-lubricating bearings

ABSTRACT

A self-lubricating bearing for use in low pressure, high frequency, small amplitude applications and methods of operating and constructing the same, the bearing having a self-lubricating liner and a counterface surface in close sliding contact therewith, the counterface surface having a surface finish of less than  20  nm and a hardness of less than in the region of 1000VPN.

This invention relates to self-lubricating bearings and moreparticularly to self-lubricating bearings for use in low pressure orstress, high-frequency, small amplitude motion applications.

Self-lubricating bearings typically comprise a housing having a linerwhich is in sliding contact with a counterface. In the case of sphericalbearings, the counterface comprises a ball and the housing is providedwith a self-lubricating liner comprising woven or meshed fibres suffusedwith a resin to hold together a quantity of PTFE or otherself-lubricating material. Such PTFE-rich liner systems are well-knownand have been used in the past, particularly in low stress, highfrequency, small amplitude motion applications for spherical bearings.The difficulties in providing self-lubricating bearings withsufficiently long life have been addressed and various solutionsproposed and implemented including the approach taken in GB2170279—theresultant bearing consisting of an extremely hard counterface surfaceconsisting of a thick and hard coating at least 50 μm thick and having asurface finish or roughness of not greater than 50 nm. GB2170279discloses the use of surface finishes leaving a roughness in the orderof 10-20 nm at best.

GB2170279 teaches the use of a counterface with both extreme hardnessand smoothness. The extreme hardness is specified as being not less than1000VPN, with the hardness preferably being at least 1100 VPN—it shouldbe noted that this hardness level corresponds to a tungsten carbidecoated material.

Low stress, high frequency, small amplitude motion applications such asground transport suspension systems and helicopter flying control androtor systems have been thought for many years to require the technologydisclosed in GB2170279—i.e. a bearing having a counterface of extremehardness and smoothness. The particular thinking behind adopting theextreme hardness in the counterface is to stop debris which includeshard materials such as particles of resin from the liner and metal fromthe counterface from damaging the surface finish of the counterface. Assoon as the surface finish of the counterface is damaged, thoseirregularities or rough areas further damage the liner creating moreliner debris resulting in increased backlash in the bearing leading to areduced bearing life.

The general thinking has, therefore, been that the counterface surfacemust be made harder than the hard debris which results in the very hardcounterface surface required in GB2170279. This requirement haspreviously necessitated the use of thick (>50 μm) coatings which canonly be machined after application. Any deviation from thecharacteristics specified in GB2170279, particularly with regard to thehardness and surface finish of the counterface, has therefore beenregarded as unadvisable if one wishes to produce a bearing with a lifecomparable with that demonstrated in GB2170279 when used in low pressureor stress, high-frequency, small amplitude motion applications.

It is an object of the present invention to provide a self-lubricatingliner which does not suffer from the requirement to use a counterface ofextreme hardness and which provides at least comparable performance andlife duration under the same operating conditions as disclosed inGB2170279.

Accordingly, one aspect of the present invention provides aself-lubricating bearing for use in low pressure, high frequency, smallamplitude applications, the bearing having a self-lubricating liner anda counterface surface in close sliding contact therewith, thecounterface surface having a surface finish of less than 20 nm and ahardness of less than in the region of 1000VPN.

Preferably, the surface finish of the counterface surface is in therange of 5 nm to 20 nm.

Conveniently, the counterface surface comprises a coating on a curvedsurface, the curved surface having an electrolytically ground finish.

Advantageously, the coating over the electrolytically ground finish hasa thickness of between 1-5 μm.

Preferably, the coating is a chemical deposition coating, a physicalvapour deposition coating or an ion plating coating.

Conveniently, the bearing is a spherical bearing.

Advantageously, the spherical bearing includes a ball, the ballproviding the counterface surface.

Preferably, the operating conditions, in use, are at stresses of lessthan 35 MPa, at a frequency of at least 0.1 Hz and with amplitudescomprising small angular motions of less than ±12° rotation.

Another aspect of the present invention provides a method ofconstructing a self-lubricating bearing comprising the steps of:providing a self-lubricating liner with a curved surface; providing acounterface having a curved surface; electrolytically grinding thecurved surface of the counterface to a surface finish of less than 20 nmto produce a counterface surface having a hardness of less than 1000VPN;and placing the curved surfaces of the liner and the counterface surfacein sliding contact with one another.

Conveniently, the curved surfaces are correspondingly curved surfaces.

A further aspect of the present invention provides a method of operatinga self-lubricating bearing having a self-lubricating liner and acounterface surface in close sliding contact therewith, the counterfacesurface having a surface finish of less than 20 nm and a hardness ofless than 1000VPN, wherein the operating conditions are at stresses ofless than 35 MPa, at a frequency of at least 0.1 Hz and with amplitudescomprising small angular motions of less than ±12° rotation.

In order that the present invention may be more readily, understood,embodiments thereof will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a cross-section of a detail of a self-lubricating bearingembodying the present invention; and

FIG. 2 is a graphical comparison of wear test results of bearingsembodying the present invention and known bearings;

FIG. 3 is a graphical representation of wear test results of bearingsembodying the present invention and known bearings.

Referring to FIG. 1, a self-lubricating spherical bearing embodying thepresent invention is particularly designed for use under the conditionsof low pressure or stress, high-frequency and small amplitude motion.Such conditions are, more particularly: at stresses of less than 35 MPa,at a frequency of at least 0.1 Hz (helicopter applications are typically5 to 30 Hz) and with amplitudes comprising small angular motions of lessthan ±12° rotation and tilt of ±5°. Such conditions are typically foundin helicopter flying control and rotor systems and ground transportsuspension systems. The self-lubricating spherical bearing embodying thepresent invention comprises a housing 1 having a spherical bearingsurface 2 upon which a liner system 3 is bonded. A ball 4 having acounterface surface 5 in close contact with the liner system 3 sits inand is typically restrained in the housing. The present invention isapplicable to other forms of self-lubricating bearings and notexclusively to spherical bearings. Examples of other forms of bearingshaving a sliding surface in sliding contact with a counterface includecylindrical journal bearings and flat contact bearings.

There is a sliding contact between the counterface 5 and the slidingsurface 6 of the liner system 3.

The liner system 3 comprises a PTFE-rich self-lubricating liner system.The liner system is a low coefficient of friction liner system such asproduced by NMB Minebea and consists of an enriched PTFE surface on aPTFE/Resin matrix, reinforced by a plain Nomex (Trade Mark) weave.

In direct contrast to the teaching of GB2170279, the counterface surface5 does not have the extreme high hardness required in GB2170279. Thecounterface surface 5 of a preferably heat treated steel alloy ball asper AMS5630, has a hardness of 56 Rc to 62 Rc (650 to 750 VPN). Further,the counterface surface 5 is prepared to have a surface roughness in theorder of 5 nm and preferably in the range of 5 nm to 20 nm.

The improved texture of the surface, the reduction in surface finishdefects, in combination with the enriched PTFE surface of the liner isextremely important to the invention since it removes the requirement,hitherto thought to be essential, of extreme hardness in the counterfacesurface as specified in GB2170279. By adopting a surface finish in therange of 5 nm to 20 nm, it has been surprisingly found that thecounterface surface 5, even when softer than the extreme hardnessesquoted in GB2170279, in combination with the liner system creates lessdamage to the counterface surface and so does not destroy the goodsurface finish of the counterface surface 5 leading to a longer lifebearing.

The most preferred method of achieving the requisite surface finish inthe range of 5 nm to 20 nm is by electrolytic grinding as set out inEP-A-1078714. This method has not previously been used to improve thesurface finish on spherical bearings. After manufacture and usingconventional bearing finishing techniques a surface finish in the regionof 40 nm would be achieved whereas the electrolytic grinding methodresults in a much improved surface finish in the order of 5 nm to 20 nm.

A further improvement in surface finish and wear properties can berealised by utilising a coating such as a physical vapour depositioncoating of between 1-5 μm over the electrolytically ground finish.Physical vapour deposition coatings could not be used with the bearingspecified in GB2170279 because the necessary hardness for thecoating—comprising the counterface surface—is not achievable at the lowcoating thicknesses produced by such deposition techniques. In contrast,because the claimed invention does not require a coating of extremehardness, chemical or vapour deposition and ion plating techniques canbe readily used. The physical vapour deposition coating gives rise to acounterface surface having a hardness in the region of 1000 VPN—stilllower than the extreme hardness quoted in GB2170279.

Test results have shown improved bearing life for bearings embodying thepresent invention compared to like-tests carried out on bearing surfaceswithout the improved surface finish of 20 nm or less. The results shownin FIG. 2 show the range of wear results achieved for two differentclasses of bearing—the conventional finish (in the region of 70 nm) andthe finish on bearings embodying the present invention which is in theregion of 5 to 20 nm.

Further, referring to FIG. 3, the wear test results are shown for thefollowing bearings:

Example 1 comprises a bearing having a heat treated steel alloy ball asper AMS5630 having a hardness of 56 Rc to 62 Rc (650 to 750 VPN)provided with a conventional surface finish (in the region of 70 nm);

Example 2 comprises a bearing having another heat treated steel alloyball as per AMS5630 having a hardness of 56 Rc to 62 Rc (650 to 750 VPN)provided with a conventional surface finish (in the region of 70 nm);

Example 3 comprises a bearing having a heat treated steel alloy ball asper AMS5630 having a hardness of 56 Rc to 62 Rc (650 to 750 VPN)provided with an electrolytically ground surface finish (of less than 20nm) with no coating;

Example 4 comprises a bearing having a heat treated steel alloy ball asper AMS5630 having a hardness of 56 Rc to 62 Rc (650 to 750 VPN)provided with an electrolytically ground surface finish (of less than 20nm) and a physical vapour deposition coating;

Example 5 comprises a bearing having another heat treated steel alloyball as per AMS5630 having a hardness of 56 Rc to 62 Rc (650 to 750 VPN)provided with an electrolytically ground surface finish (of less than 20nm) and a physical vapour deposition coating;

Example 6 comprises a bearing having a ball manufactured in accordancewith GB2170279 with no coating; and

Example 7 comprises a bearing having a ball with a tungsten carbidecoating manufactured in accordance with GB2170279.

It should be noted that only Examples 3, 4 and 5 comprise embodiments ofthe present invention.

In the present specification “comprises” means “includes or consists of”and “comprising” means “including or consisting of”.

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, as appropriate,may, separately, or in any combination of such features, be utilised forrealising the invention in diverse forms thereof.

1. A self-lubricating bearing for use in low pressure, high frequency,small amplitude applications, the bearing having a self-lubricatingliner and a counterface surface in close sliding contact therewith, thecounterface surface having a surface finish of less than 20nm and ahardness of less than in the region of 1000VPN.
 2. A self-lubricatingbearing according to claim 1, wherein the surface finish of thecounterface surface is in the range of 5 nm to 20 nm.
 3. Aself-lubricating beating according to claim 1, wherein the counterfacesurface comprises a coating on a curved surface, the curved surfacehaving an electrolytically ground finish.
 4. A self-lubricating bearingaccording to claim 3, wherein the coating over the electrolyticallyground finish has a thickness of between 1-5 μm.
 5. A self-lubricatingbearing according to claim 3, wherein the coating is a chemicaldeposition coating, a physical vapour deposition coating or an ionplating coating.
 6. A self-lubricating bearing according to claim 1,wherein the beating is a spherical bearing.
 7. A self-lubricatingbearing according to claim 6, wherein the spherical bearing includes aball, the ball providing the counterface surface.
 8. A self-lubricatingbearing according to claim 1, wherein the operating conditions, in use,are at stresses of less than 35 MPa, at a frequency of at least 0.1 Hzand with amplitudes comprising small angular motions of less than ±12°rotation.
 9. A method of constructing a self-lubricating bearingcomprising the steps of: providing a self-lubricating liner with acurved surface; providing a counterface having a curved surface;electrolytically grinding the curved surface of the counterface to asurface finish of less than 20 nm to produce a counterface surfacehaving a hardness of less than 1000VPN; and placing the curved surfacesof the liner and the counterface surface in sliding contact with oneanother.
 10. A method according to claim 9, wherein the curved surfacesare correspondingly curved surfaces.
 11. A method of operating aself-lubricating bearing having a self-lubricating liner and acounterface surface in close sliding contact therewith, the counterfacesurface having a surface finish of less than 20 nm and a hardness ofless than 1000VPN, wherein the operating conditions are at stresses ofless than 35 MPa, at a frequency of at least 0.1 Hz and with amplitudescomprising small angular motions of less than ±12° rotation. 12.(canceled)
 13. A method according to claim 9, wherein the bearing is aspherical bearing comprising a ball, and the counterface is a surface ofthe ball.
 14. A method according to claim 9, further comprising forminga coating on the curved surface of the counterface, wherein the coatingprovides the counterface surface in sliding contact with the liner. 15.A method according to claim 14, wherein the coating is a chemicaldeposition coating, a physical vapour deposition coating or an ionplating coating.
 16. A method according to claim 9, wherein thecounterface surface has a hardness of less than 750VPN.
 17. A methodaccording to claim 14, wherein the coating has a thickness in the rangeof about 1-5 μm.
 18. A method according to claim 11, wherein the bearingis a spherical bearing comprising a ball, and the counterface surface isa surface of the ball.
 19. A method according to claim 18, wherein theball is a heat treated metal ball.
 20. A method according to claim 19,wherein the ball has a physical vapour deposition coating providing thecounterface surface in sliding contact with the liner.